Tire Sidewall for a Heavy Duty Civil Engineering Vehicle

ABSTRACT

A radial tire ( 10 ) for a heavy vehicle of construction plant type, and more particularly, the sidewalls thereof ( 20 ), arranged to minimize the temperature of the tire while guaranteeing its electrical conductivity. The tread ( 30 ) comprises two tread wings ( 31 ) and a central portion ( 32 ). The bead layer ( 71 ), the elastomeric coating compound of the carcass layer ( 50 ), the second sidewall layer ( 22 ) and the tread wing ( 31 ) constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground.

The present invention relates to a radial tire intended to be fitted toa heavy vehicle of construction plant type, and more particularly to thesidewalls of such a tire.

A radial tire for a heavy vehicle of construction plant type is intendedto be mounted on a rim, the diameter of which is at least equal to 25inches, according to European Tire and Rim Technical Organisation orETRTO standard. It is usually fitted to a heavy vehicle, intended tobear high loads and to run on harsh terrain such as stone-coveredtracks.

Generally, since a tire has a geometry of revolution relative to an axisof rotation, its geometry is described in a meridian plane containingits axis of rotation. For a given meridian plane, the radial, axial andcircumferential directions respectively denote the directionsperpendicular to the axis of rotation, parallel to the axis of rotationand perpendicular to the meridian plane.

In the following text, the expressions “radially inner/radially on theinside” and “radially outer/radially on the outside” mean “closer to”and “further away from the axis of rotation of the tire”, respectively.“Axially inner/axially on the inside” and “axially outer/axially on theoutside” mean “closer to” and “further away from the equatorial plane ofthe tire”, respectively, the equatorial plane of the tire being theplane passing through the middle of the running surface andperpendicular to the axis of rotation.

The top end of a component of the tire refers to the radially outer endof said component. Conversely, the bottom end refers to the radiallyinner end of said component.

A tire comprises a tread intended to come into contact with the ground,the two axial ends of which are connected via two sidewalls to two beadsthat provide the mechanical connection between the tire and the rim onwhich it is intended to be mounted.

A radial tire further comprises a reinforcement made up of a crownreinforcement radially on the inside of the tread and a carcassreinforcement radially on the inside of the crown reinforcement.

The crown reinforcement of a radial tire comprises a superposition ofcircumferentially extending crown layers radially on the outside of thecarcass reinforcement. Each crown layer is made up of generally metallicreinforcers that are mutually parallel and coated in a polymericmaterial of the elastomer or elastomeric compound type.

The carcass reinforcement of a radial tire customarily comprises atleast one carcass layer comprising generally metallic reinforcers thatare coated in an elastomeric compound. A carcass layer comprises a mainpart that joins the two beads together and is generally wound, in eachbead, from the inside of the tire to the outside around a usuallymetallic circumferential reinforcing element known as a bead wire so asto form a turn-up. The metallic reinforcers of a carcass layer aresubstantially parallel to one another and form an angle of between 85°and 95° with the circumferential direction.

A tire sidewall comprises at least one sidewall layer consisting of anelastomeric compound and extending axially towards the inside of thetire from an outer face of the tire, in contact with the atmosphericair. At least in the region of greater axial width of the tire, thesidewall extends axially inwardly to an axially outermost carcass layerof the carcass reinforcement.

An elastomeric compound is understood to mean an elastomeric materialobtained by blending its various constituents. An elastomeric compoundconventionally comprises an elastomeric matrix comprising at least onediene elastomer of the natural or synthetic rubber type, at least onereinforcing filler of the carbon black type and/or of the silica type, ausually sulfur-based crosslinking system, and protective agents.

An elastomeric compound may be characterized mechanically, in particularafter curing, by its dynamic properties, such as a dynamic shear modulusG*=(G′²+G″²)^(1/2), wherein G′ is the elastic shear modulus and G″ isthe viscous shear modulus, and a dynamic loss tgδ=G″/G′. The dynamicshear modulus G* and the dynamic loss tgδ are measured on a viscosityanalyser of the Metravib VA4000 type according to standard ASTM D5992-96. The response of a sample of vulcanized elastomeric compound inthe form of a cylindrical test specimen with a thickness of 4 mm and across section of 400 mm², subjected to a simple alternating sinusoidalshear stress, at a frequency of 10 Hz, with a deformation amplitudesweep from 0.1% to 50% (outward cycle) and then from 50% to 0.1% (returncycle), at a given temperature, for example equal to 60° C., isrecorded. These dynamic properties are thus measured for a frequencyequal to 10 Hz, a deformation equal to 50% of the peak-to-peakdeformation amplitude, and a temperature that may be equal to 60° C. or100° C.

An elastomeric compound may also be characterized by its electricalresistivity which characterizes the ability of the compound to let theelectrical charges move freely, and therefore to allow the flow of anelectrical current. The electrical resistivity is generally denoted byρ, and its unit of measurement is in Ohm.metre (Ω·m) but it is common,in the field of tires, to express the measurement of the electricalresistivity in Ohm.centimetre (Ω·cm). The test for measurement of theelectrical resistivity is described, for example, in the standardASTM-D257. An electrical resistivity of 1 Ω·m, or of 10²′Ω·cm,corresponds to the resistance to the flow of the electric current in acylindrical portion of compound having a length of 1 m and a crosssection of 1 m². The electrical conductivity is the inverse of theelectrical resistivity, denoted by σ and satisfying σ=1/ρ. Subsequently,use will be made of either the electrical conductivity σ or theelectrical resistivity ρ, depending on the context, to characterize theelectrical properties of the compounds.

A material that is very weakly electrically conductive or that iselectrically resistant is understood to mean a material having anelectrical resistivity of greater than 10⁸ Ω·cm. Similarly, anelectrically conductive material is understood to mean a material havinga resistivity of less than 10⁶′Ω·m. These materials may or may not beelastomeric compounds.

The electrical resistivity properties of the elastomeric compounds aredirectly linked to their composition and in particular to the use ofreinforcing fillers. It is known that an amount of from 35 to 45 phr(parts per hundred parts of elastomer) of carbon black is sufficient togive an elastomeric compound a resistivity sufficient to dischargeelectrostatic charges.

It is also known that a combination of reinforcing fillers of carbonblack type and of silica type, in suitable proportions, favours thereaching of a performance compromise between the rolling resistance andthe endurance of the tire, by lowering the temperature level. However,if the amount of carbon black is less than 35 phr, the elastomericcompound is electrically insulating.

By way of illustration, a tread elastomeric compound with a reinforcingfiller comprising at least 40 phr of silica, and at most 10 phr ofcarbon black has an electrical resistivity of the order of 10¹²′Ω·m.

Furthermore, the thermal conductivity or conductibility of a material isa physical quantity that characterizes the ability of the material toallow heat transfer by conduction. It represents the amount of heattransferred per unit of area and of time, under a temperature gradientof 1 degree Kelvin or 1 degree Celsius and per metre. In theInternational System of Units, the thermal conductivity is expressed inwatt per metre.Kelvin (W·m⁻¹.K⁻¹).

Thus, a thermal conductivity of 1 W·m⁻¹K⁻¹ represents the amount of heatwhich propagates through a material by thermal conduction, across anarea of 1 m², over a distance of 1 m. The measurement of the thermalconductivity on a test specimen of elastomeric compound is described,for example, in the standard ASTM-F433.

Just like for the electrical conductivity, the thermal conductivity isdirectly linked to the composition of the elastomeric compounds. Theheat transfer by conduction is carried out by virtue of the reinforcingfillers. Thus, by way of illustration, a tread elastomeric compoundcomprising a reinforcing filler comprising at least 40 phr of silica andat most 10 phr of carbon black is an electrically insulating compound,that is endowed with a low thermal conductivity.

With the aim of improving the rolling resistance and therefore ofreducing the fuel consumption, the tires on the market often compriseelastomeric compounds predominately comprisingnon-electrically-conductive reinforcing fillers such as silica, or elseelastomeric compounds weakly loaded with electrically-conductivereinforcing filler such as carbon black.

The use of these elastomeric compounds has thus been widely developedfor the creation of treads given the advantages afforded by suchcompounds in also improving the performance relating to the grip on dry,wet or icy ground, the wear resistance, or else the running noise. Thistype of tire is described by way of illustration in European patentapplication EP-501 227.

However, the use of these elastomeric compounds is accompanied by adifficulty linked to the accumulation of static electricity when thevehicle is running, and to the absence of flow of these charges to theground due to the very high resistivity of the elastomeric compoundsconstituting said tread. The static electricity thus accumulated in atire is capable of causing, when certain particular conditions are met,an electric shock to the occupant of a vehicle, when he or she has totouch the body of the vehicle. This static electricity is, moreover,capable of accelerating the aging of the tire due to the ozone generatedby the electrical discharge. It may also be the cause, depending on thenature of the ground and the vehicle, of a poor operation of thebuilt-in radio in the vehicle due to the interferences that itgenerates.

This is the reason why many technical solutions have been proposed toenable the flow of the electrical charges between the crown of the tireand the ground.

However, these known technical solutions usually consist in connectingthe tread to a portion of the tire, such as the sidewall, a crownreinforcement layer or a carcass reinforcement layer, which haselectrically-conductive properties. The electric charges are thereforedischarged to the ground from the rim, connected to the vehicle, bysuccessively passing through the bead of the tire in contact with therim, the sidewalls and more particularly the elastomeric coatingcompounds of the carcass layer reinforcers or at least one sidewallelastomeric compound, and finally the crown reinforcement and the tread.

The thermomechanical study of a tire for a construction plant vehicleshows that the viscoelastic losses of the elastomeric compounds aresources of heat, the intensity of which depends on the volume of theelastomeric compounds and on the deformations that they undergo. Thisheat, which is generated when the tire is in motion, is discharged intothe environment more or less quickly depending on the thermalconductivity values of each material of the tire. For an elastomericcompound, when its thermal conductivity is too low, the heat accumulatesand results in the bakelization thereof. The tire then loses its elasticproperties, which is unfavourable for the use thereof.

Thus, the optimization of the endurance of a tire for a constructionplant vehicle requires maintaining the operating temperature at asuitable level. The control of the temperature level depends on thecomposition of the elastomeric compounds, and especially on the amountof reinforcing fillers. Ultimately, the optimization of the endurance ofthe tire leads to a coupled problem where the physical parametersinvolved are the viscous shear modulus, or the viscoelastic loss, whichis directly connected to the viscoelastic heat sources, the thermalconductivity which controls the conduction of the heat in theelastomeric compounds, and the electrical conductivity which must be ata level sufficient for discharging electrostatic charges.

In a tire for a construction plant vehicle, the tread represents around35% to 40% of the total volume of rubber of the tire, and the sidewallsaround 15% of this same volume. The tread being subjected to the shearstresses of the ground is the site of large-amplitude strains. Asregards the sidewalls which are subjected to bending cycles during theuse of the tire, the shear strains are also sizeable. The inventors havetherefore focused on these two zones of high mechanical stresses inorder to determine the optimal compositions of the elastomeric compoundsto meet the desired performance compromise between minimal thermal leveland ability to discharge the electrostatic charges.

The inventors have thus set themselves the objective of improving theendurance of a tire for a construction plant vehicle, limiting itsaverage operating temperature to an appropriate level of around 92° C.,while guaranteeing its ability to be electrically conductive, i.e. todischarge the electrostatic charges.

This objective has been achieved by a tire for a heavy vehicle ofconstruction plant type, comprising:

-   -   a tread comprising two axial end portions or tread wings axially        separated by a central portion;    -   two sidewalls connecting the tread wings to two beads, intended        to come into contact with a mounting rim by means of a bead        layer made of electrically-conductive elastomeric compound;    -   each sidewall being axially on the outside of a carcass        reinforcement comprising at least one carcass layer consisting        of metallic reinforcers that are coated in an        electrically-conductive elastomeric coating compound;    -   each sidewall consisting of a laminate comprising at least two        sidewall layers that are at least partly axially superposed and        having a total thickness E;    -   the axially outermost first sidewall layer having a thickness E₁        and consisting of a first elastomeric compound M₁;    -   the first elastomeric compound M₁ having a viscous shear modulus        G″₁ and a thermal conductivity λ₁;    -   the axially innermost second sidewall layer having a thickness        E₂ and consisting of a second elastomeric compound M₂;    -   the second elastomeric compound M₂ having a viscous shear        modulus G″₂, a thermal conductivity λ₂ and an electrical        resistivity ρ₂;    -   each tread wing consisting of a third elastomeric compound M₃        having an elastic dynamic shear modulus G′₃ and an electrical        resistivity ρ₃;    -   the thickness E₁ of the first sidewall layer being at least        equal to 0.9 times the total thickness E of the laminate;    -   the thickness E₂ of the second sidewall layer being at least        equal to the minimum value between 3 mm and 0.1 times the total        thickness E of the laminate;    -   the first elastomeric compound M₁ of the first sidewall layer        having a viscous shear modulus G″₁ at most equal to 0.165 MPa        and a thermal conductivity λ₁ at least equal to 0.190 W/m·K;    -   the second elastomeric compound M₂ of the second sidewall layer        having a viscous shear modulus G″₂ at most equal to 0.3 MPa and        a thermal conductivity λ₂ greater than the thermal conductivity        λ₁ of the compound M₁ of the first sidewall layer;    -   and the electrical resistivities ρ₂ and ρ₃ respectively of the        second elastomeric compound M₂ of the second sidewall layer and        of the third elastomeric compound M₃ of the tread wing are at        most equal to 10⁶′Ω·cm, so that the bead layer, the elastomeric        coating compound of the carcass layer, the second sidewall layer        and the tread wing constitute a preferential conductive pathway        of the electric charges between the rim and the ground when the        tire is mounted on its rim and flattened on the ground.

The essential idea of the invention is to simultaneously optimize thedesign of the sidewalls of the tire and that of its tread which isdivided into three portions: a central portion, and two tread wingslocated axially on either side of the central portion. Each sidewallconsists of a laminate of two axially superposed layers of elastomericcompounds. The invention relates both to the geometry and the physicalproperties of the elastomeric compounds of the tread and of thetwo-layer sidewall laminate.

According to the invention, regarding the geometry, the thickness E₁ ofthe axially outer first sidewall layer is at most less than 0.9 timesthe total thickness E of the laminate, and the thickness E₂ of theaxially inner second sidewall layer is at least equal to the minimumvalue between 3 mm and 0.1 times the total thickness E of the laminate.

The sidewall consists of an axially outermost first layer of elastomericcompound, intended to be in contact with the atmospheric air. For aconstruction plant tire, this first layer has a relatively greatthickness E₁, typically of the order of 35 mm. The axially innermostsecond sidewall layer is in contact with the elastomeric coatingcompound of the carcass layer, and has a relatively small thickness E₂at most equal to the minimum value between 3 mm and 0.1 times the totalthickness E of the laminate. Combined with the axially outer firstsidewall layer is a low-hysteresis elastomeric compound, whereas, forthe axially inner second sidewall layer, the elastomeric compound isoptimized relative to its electrical resistivity and thermalconductivity properties. This second elastomeric compound of the secondsidewall layer is a link in the pathway for discharging electrostaticcharges from the tread wing in contact with the ground, via theelastomeric coating compound of the carcass layer, to the bead layer ofthe tire which is in contact with the rim.

It should be noted that, preferentially, the sidewall is formed by alaminate comprising only two sidewall layers, but that a laminate havingmore than two layers can also be envisaged, or else a single sidewallmade of a single layer of low-hysteresis elastomer that is sufficientlyelectrically conductive. The mechanisms disclosed in the presentdocument are however described in the case of a two-layer laminate.

Also according to the invention, the viscous shear modulus G″₁ and thethermal conductivity λ₁ of the first elastomeric compound M₁ of theaxially outer first sidewall layer are defined such that G″₁ is at mostequal to 0.165 MPa, and λ₁ is at least equal to 0.190 W/m·K.

In the sidewall, the tire works at imposed strains, and the viscousshear modulus controls the temperature level. The composition of thefirst elastomeric compound of the first sidewall layer thus aims tominimize the value of the viscous shear modulus with a value at mostequal to 0.165 MPa. The distribution of the respective thicknesses ofthe two sidewall layers is carried out so that the sidewall layer whichhas the lowest hysteresis, with a maximum viscous shear modulus G″₁ of0.165 MPa and a maximum dynamic loss of 0.150, has the greatestthickness and is positioned on the outer side of the tire. Thecorresponding thermal conductivity, with a minimum value of 0.190 W/m·K,enables conductive transfer to the outer periphery of the tire, inaddition to the heat exchange flows thus guaranteeing the discharging ofthe heat and the maintaining of the temperature of the first sidewalllayer at an appropriate temperature.

Again according to the invention, the electrical resistivity ρ₂ of thesecond elastomeric compound M₂ of the second sidewall layer is less thanor equal to 10⁶′Ω·cm, and its thermal conductivity λ₂ is greater thanthe thermal conductivity of the compound M₁ of the first sidewall layer.

The composition of the second elastomeric compound M₂ of this secondsidewall layer should above all be electrically conductive. The value ofits electrical resistivity ρ₂ should be at most equal to 10⁶′Ω·cm. Thisaxially inner second sidewall layer has relatively higher hysteresisthan the axially outer first sidewall layer, with a viscous shearmodulus G″₂ of 0.3 MPa. But the volume thereof is significantly smallerwith a thickness corresponding to the minimum value between 3 mm and atenth of the total thickness of the laminate. Considering the value ofits electrical resistivity, the level of its thermal conductivity atleast equal to 0.240 W/m·K favours the transfer of heat from the carcassreinforcement to the axially outer first sidewall layer.

Still according to the invention, the electrical resistivities ρ₂ and ρ₃respectively of the elastomeric compounds M₂ of the second sidewalllayer and M₃ of the tread wing are less than or equal to 10⁶′Ω·cm, sothat the bead layer, the elastomeric coating compound of the carcasslayer, the second sidewall layer and the tread wing constitute apreferential conductive pathway of the electric charges between the rimand ground when the tire is mounted on its rim and flattened on theground.

Advantageously, the second sidewall layer is in contact via a radiallyouter top end with a tread wing over a length L_(h) at least equal to 10mm.

Again advantageously, the second sidewall layer is in contact via aradially inner bottom end with the elastomeric coating compound of thecarcass layer over a length L_(b) at least equal to 10 mm.

The objective of obtaining an electrically conductive tire results fromthe correct operation of the pathway for discharging the electrostaticcharges. The interfaces of the various constituents of the pathway fordischarging the electrostatic charges must be in contact, in twos, overa length of at least 10 mm, so as to always guarantee the continuity ofthe pathway for discharging the electrostatic charges to take intoaccount the manufacturing tolerances.

According to the inventors, the thermal conductivity λ₂ of theelastomeric compound of the second sidewall layer is greater than orequal to 0.240 W/m·K. Thus, in addition to the anticipated electricalconductivity properties of this compound, with this level of thermalconductivity, it helps to discharge the heat by conduction from theinside of the tire to the outside.

The elastomeric compound M₃ of the tread wing advantageously has anelastic shear modulus G′₃ at least equal to 1.4 MPa. Specifically, theelastomeric compound M₃ of the tread wing is in contact with the groundand, consequently, must be compatible with the grip and wear performancerequirements since the elastomeric compound M₃ is under circumferentialand transverse shear stress.

The elastomeric compound M₃ of the tread wing also advantageously has athermal conductivity λ₃ at least equal to 0.240 W/m·K, guaranteeing theconduction of heat from the inside of the tire to the running surfacethereof. The discharging of the heat is carried out by conduction in therunning surface in contact with the ground, and by convection on theouter periphery of the tire not in contact with the ground, by means ofthe surfaces delimited by the tread pattern of the tire.

According to one preferred embodiment of the tread wing, the thirdelastomeric compound M₃ of at least one tread wing is anelectrically-conductive rubber composition based at least onpolyisoprene, on a crosslinking system and on at least one reinforcingfiller comprising carbon black, characterized by a BET surface area atleast equal to 110 m²/g and by a content at least equal to 30 phr and atmost equal to 80 phr.

The tread wings consist of an elastomeric compound intended to be incontact with the ground. In addition to the anticipated electricalproperties, the composition of the elastomeric compound should becompatible with the grip and wear performance requirements of the tire.The tread wings thus have a sufficient thickness to be in contact withthe ground throughout the service life of the tire. The reinforcingfillers of this elastomeric compound are in a sufficient amount, with acarbon black content of from 30 to 80 phr, and of appropriate quality,with a BET surface area of greater than 110 m²/g, to guarantee theelectrical conductivity thereof. As is known, the BET specific surfacearea of carbon blacks is measured according to the standard D6556-10[multipoint method (at least 5 points)—gas: nitrogen—P/P0 relativepressure range: 0.1 to 0.3]. The thermal conductivity is simultaneouslyadjusted to a level sufficient to ensure the transfer of heat byconduction to the running surface of the tire. For example, a thermalconductivity value equal to 0.240 W/m·K is suitable. The thermaltransfer of the heat of the tread is also carried out by convection atthe outer surface of the tire which is not in contact with the ground.

Preferentially, the two tread wings are formed by such an elastomericcompound, but, if a single tread wing is formed by such an elastomericcompound, the desired technical effect is also present. In other words,the solution proposed by the invention still remains valid for tireswhich would have a tread that is nonsymmetrical relative to theequatorial plane, with tread wings consisting of different elastomericcompounds. The presence of the pathway for discharging the electrostaticcharges on a single side of the tire is in principle sufficient.

According to one preferred embodiment of the axially inner secondsidewall layer, the second elastomeric compound M₂ of the axially innersecond sidewall layer is an electrically-conductive rubber compositionbased at least on a mixture of polyisoprene and polybutadiene, on acrosslinking system, and on a reinforcing filler comprising carbonblack, characterized by a BET surface area at least equal to 80 m²/g andby a content at least equal to 40 phr and at most equal to 60 phr.

The main role of the axially inner second sidewall layer is to ensurethe continuity of the pathway for discharging the electrostatic chargesbetween the tread and the bead layer. The composition of the elastomericcompound should thus contain an amount of reinforcing filler sufficientto guarantee the electrical conductivity. This property is obtained, forexample, with an amount of from 40 to 60 phr of carbon black, combinedwith an elastomer based on a mixture of polyisoprene and polybutadiene.The carbon black fillers furthermore have a BET surface area at leastequal to 80 m²/g. At the same time, the thermal conductivity is improvedthereby and this layer of elastomeric compound participates in thecontrol of the temperature level of the tire by favouring thedischarging of the heat from the inside to the outside of the tire.

According to one preferred embodiment of the axially outer firstsidewall layer, the first elastomeric compound M₁ of the axially outerfirst sidewall layer (21) has a rubber composition based on at least oneblend of polyisoprene, natural rubber or synthetic polyisoprene, andpolybutadiene, on a crosslinking system, and on a reinforcing filler, atan overall content at most equal to 45 phr, and comprising carbon black,at a content at most equal to 5 phr, and, predominantly, silica, at acontent at least equal to 20 phr and at most equal to 40 phr.

On this axially outer portion of the sidewall, the composition of theelastomeric compound should lead to a reduction in the hysteresis.However, this drop in the hysteresis should be able to be achievedwithout deteriorating, in particular, the mechanical properties such asthe fatigue strength and, more particularly, the crack resistance.Indeed, the sidewalls of a construction plant tire are subjected to veryhigh stresses simultaneously in terms of bending strain, attacks andthermal stresses. These prolonged static or dynamic stresses of thesidewalls, in the presence of ozone, cause more or less pronouncedcrazing or cracks to appear, the propagation of which under the effectof the stresses may give rise to significant damage of the sidewall inquestion. It is therefore important for the elastomeric compoundsconstituting the tire sidewalls, for construction plant tires inparticular, to have very good mechanical properties, imparted inparticular by a high content of reinforcing fillers.

According to a first embodiment of the central tread portion, formed bya fourth elastomeric compound M₄, the fourth elastomeric compound M₄ ofthe central tread portion is a rubber composition based on at least onediene elastomer, on a crosslinking system, and on a reinforcing fillercomprising carbon black, characterized by a BET surface area at mostequal to 115 m²/g and by a content at most equal to 40 phr, and silica,at a content at most equal to 20 phr. Advantageously, the mixture of theelastomer and carbon black is obtained beforehand via a liquid route.

In a construction plant tire, the tread represents around 40% of thetotal volume of rubber and is, in fact, the main source of hysteresis.To improve the endurance, one of the solutions consists in obtainingelastomeric compounds of very low hysteresis in order to limit thetemperature level. By being free of the electrical resistivityconstraint for this elastomeric compound of the tread, in particular inthe central portion thereof, the composition may focus on the reductionof the hysteresis, using, for example reinforcing fillers made of carbonblack and of silica in an elastomer obtained via a liquid route. To dothis, use is made of an elastomer in latex form in the form of elastomerparticles dispersed in water, and of an aqueous dispersion of thefiller, i.e. a filler dispersed in water, commonly referred to as“slurry”. Thus, a viscoelastic dynamic loss characterized by tg(δ_(max)) of the order of 0.06, measured at 100° C. and for a stressfrequency of 10 Hz, is obtained. The elastomeric compound of the centraltread portion consequently has a low hysteresis while having compatibleproperties for the wear and grip performance.

According to a second embodiment of the central tread portion, formed bya fourth elastomeric compound M₄, the fourth elastomeric compound M₄ ofthe central tread portion is a rubber composition based on at least onediene elastomer, on a crosslinking system, and on a reinforcing filler,at an overall content at most equal to 40 phr, and comprising carbonblack, and silica.

This alternative composition of the elastomeric compound of the centraltread portion meets the same requirement of minimizing the hysteresiswhile retaining properties in order to guarantee the grip and wearperformance.

Lastly, according to a third embodiment of the central tread portion,formed by a fourth elastomeric compound M₄, the fourth elastomericcompound M₄ of the central tread portion is an electrically-conductiverubber composition based on at least one diene elastomer, on acrosslinking system, and on a reinforcing filler comprising carbonblack, characterized by a BET surface area at least equal to 120 m²/gand by a content at least equal to 35 phr and at most equal to 80 phr,and silica, at a content at most equal to 20 phr.

The presence of a pathway for discharging the electrostatic charges aspresented by the invention remains compatible with the use, in thecentral portion of the tread, of an electrically-conductive elastomericcompound. The compounds mainly filled with carbon black in amounts offrom 30 to 80 phr, and with a BET surface area of greater than or equalto 120 m²/g fall under this category.

The architecture of the tire according to the invention will be betterunderstood with reference to FIG. 1, not to scale, which represents ameridian half section of a tire.

FIG. 1 schematically represents a tire 10 intended to be used on Dumpertype vehicles. FIGS. 2, 3, and 4 represent the various possibleconfigurations of the tread wings relative to the central portion.

In FIG. 1, the tire 10 comprises a radial carcass reinforcement 50,anchored in two beads 70 and turned up, in each bead, around a bead wire60. Each bead 70 comprises a bead layer 71 intended to come into contactwith a rim flange. The carcass reinforcement 50 is generally formed of asingle carcass layer, consisting of metal cords coated in an elastomericcoating compound. Positioned radially on the outside of the carcassreinforcement 50 is a crown reinforcement (not referenced), itselfradially on the inside of a tread 30. The tread 30 comprises, at eachaxial end, an axial end portion or tread wing 31, axially on the outsideof a central tread portion 32. Each tread axial end portion 31 isconnected to a bead 70 via a sidewall 20.

Each sidewall 20 consists of a laminate comprising two sidewall layers(21, 22) that are at least partly axially superposed and having a totalthickness E. The axially outer first sidewall layer 21 has a thicknessE₁ and the axially inner second sidewall layer 22 has a thickness E₂.

The thicknesses E₁ and E₂ respectively of the first and second sidewalllayers 21 and 22, constituting the sidewall 20, are measured along thedirection normal to the carcass reinforcement 50, defined by the axis80, in the middle of the height of the sidewall. The sidewall height ofa tire for a construction plant vehicle is standardized and defined, forexample, in the ETRTO (European Tires and Rim Organisation) manual. Themeasurement points correspond to the positions determined by theintersections of the axis 80 with the faces of said sidewall layers.

According to the invention, the thickness E₁ of the first sidewall layer21 is at most less than 0.9 times the total thickness E of the laminate,and the thickness E₂ of the second sidewall layer 22 is at least equalto the minimum value between 3 mm and 0.1 times the total thickness E ofthe laminate.

The radially outer top end 221 of the axially inner second sidewalllayer 22 is advantageously in contact with the tread wing over a lengthL_(h) at least equal to 10 mm. In the same way, its radially innerbottom end 222 is also advantageously in contact with the elastomericcoating compound of the carcass layer 50 over a length L_(b) at leastequal to 10 mm.

Similarly, the radially outer top end of the axially outer firstsidewall layer 21 is in contact with the axially inner second sidewalllayer 22. Its radially inner bottom end is in contact with the beadlayer 71. Here too, the contact lengths are at least equal to 10 mm.

The radially outer top end of the tread wing 31 is in contact with thecentral tread portion 32 over its entire thickness. Its radially innerbottom end is in contact with the axially inner second sidewall layer 22over a length at least equal to 10 mm.

The objective is to ensure a permanent contact between theelectrically-conductive elastomeric compounds, in twos, in order toguarantee the continuity of the pathway for discharging theelectrostatic charges, taking into account the manufacturing tolerances.

FIG. 2 represents a tread that is symmetrical relative to the equatorialplane comprising two axial end portions or tread wings that are actuallyseparated by a central portion. The inner end of the tread wing, in theaxial direction, is located at a given distance L₁ relative to theequatorial plane. The other outer end of the tread wing, still in theaxial direction, is positioned at a distance of L₂ of the sameequatorial plane. The reference 100 from FIG. 2 represents the exteriorside of the vehicle when the tire is mounted on this vehicle and thereference 110 represents the interior side of the vehicle.

FIGS. 3 and 4 represent a tread that is not symmetrical relative to theequatorial plane. In FIG. 3, the tread wing is positioned only on thevehicle exterior side (reference 100), and in FIG. 4, it is positionedonly on the vehicle interior side (reference 110).

The invention has more particularly been studied on a tire for a Dumpertype vehicle, of dimensions 59/80 R63, comprising, in accordance withthe invention, comprises a sidewall consisting of two sidewall layers,and a tread comprising two tread wings that are axially separated by acentral portion.

The results calculated on the tire produced according to the inventionare compared to those obtained for a reference tire of the samedimensions, comprising a sidewall consisting of a single sidewall layer,and a tread made of a single portion. The elastomeric compoundsassociated with the sidewall and with the tread of the reference tireare of standard composition for a person skilled in the art.

The inventors have established the connection between the chemicalcomposition of the elastomeric compounds and the physical parameterssuch as the electrical resistivity, the thermal conductivity, and theviscoelastic loss. By way of example, represented on the graph from theappended FIG. 5, for the two elastomeric compounds of the sidewall, arethe curves of thermal conductivities as a function of the amount ofreinforcing fillers in phr. These curves show that the elastomericcompound of the axially outer first sidewall layer filled with silica isoptimized for the hysteresis, but with a thermal conductivity that isrelatively lower than the elastomeric compound of the axially innersecond sidewall layer filled with carbon black, for which the electricalconductivity property is favoured.

According to the curve from FIG. 5, for a given content of filler, forexample carbon black, it is possible to predict the value of the thermalconductivity of the elastomeric compound. The thermal conductivities aremeasured at an ambient temperature of from 23° C. to 25° C. Thedependency of the thermal conductivity relative to the temperature isnot taken into account here.

The inventors determined the composition of the elastomeric compounds,constituting the sidewall layers, by finding a compromise between thefollowing physical parameters:

-   -   the dynamic viscoelastic loss or the viscous shear modulus which        are directly connected to the viscoelastic heat sources;    -   the thermal conductivity which controls the thermal conduction        of the heat in the compounds;    -   the electrical conductivity which must be at a level sufficient        for discharging electrostatic charges.

In the example studied, the compositions of the elastomeric compounds,resulting from this compromise, are summarized in Table 1 below:

TABLE 1 Elastomeric Elastomeric compound M₁ compound M₂ Elastomeric ofthe axially of the axially Elastomeric compound M₄ outer first innersecond compound M₃ of the central Composition sidewall layer sidewalllayer of the tread wing tread portion Elastomer NR 50 50 100 100*  (Natural Rubber) Elastomer BR 50 50 NC NC (Butadiene Rubber) Carbonblack N330 NC 55 NC NC Carbon black N234 3 NC 35 35*   Silica (2) 29 NC10 10   Plasticizer (3) 10 18 NC NC Wax 1 1 NC NC Antioxidant 3 3 3 3  ZnO 2.5 2.5 2.7 2.7 Stearic acid 1 1 2.5 2.5 Sulfur 1 0.9 1.25  1.25Accelerator 0.8 0.6 1.4 1.4 * elastomeric compound M4 obtained via aliquid route (2) “Zeosil 1165MP” silica sold by Rhodia (3) “Vivatec 500”TDAE oil from Klaus Dahleke

Table 2 brings together the physical parameters of the elastomericcompounds, measured on test specimens and resulting from choices ofchemical composition:

TABLE 2 Elastomeric Elastomeric compound M₁ compound M₂ Elastomeric ofthe axially of the axially Elastomeric compound M₄ outer first innersecond compound M₃ of the central Composition sidewall layer sidewalllayer of the tread wing tread portion Thermal 0.208 0.265 0.240 0.240conductivity at 25° C. (W/m.K) Electrical 11.6 4.4 5.7 10.4 resistivityin Log (Ω.cm) Viscous shear 0.125 0.300 NC NC modulus G″max at 60° C.and 10 Hz (in MPa) Elastic shear NC NC 1.33 1.16 modulus G*max (50%,100° C. and 10 Hz) Dynamic loss NC NC 0.10 0.06 tgδ_(max) (50%, 100° C.and 10 Hz)

In a construction plant tire, the amount of elastomeric compound of thetread represents around 35% to 40% of the total mass of elastomericcompounds of the tire. The tread is thus one of the main sources ofhysteresis, and it therefore contributes greatly to the increase intemperature of the tire. Consequently, the elastomeric compound M₄ ofthe central tread portion is designed to have a low hysteresis with adynamic viscoelastic loss of the order of 0.06, measured at atemperature of 100° C., and at a frequency of 10 Hz.

In one preferred embodiment of the invention, the elastomeric compoundM₄ of the central tread portion has a composition which comprises atleast one diene elastomer and a reinforcing filler consisting of carbonblack and silica, so that the carbon black has a content at most equalto 40 phr and a BET surface area at most equal to 115 m²/g and thesilica has a content at most equal to 20 phr. The elastomer and carbonblack mixture is obtained beforehand preferentially via a liquid route.In this embodiment, the central tread portion is electricallyinsulating. The discharging of the electrostatic charges is then carriedout along the conduction pathway defined by the invention which passesthrough the tread wings in contact with the ground and which are alwayselectrically conductive.

For the elastomeric compound M₃ of the running tread wings, the overallfiller content being 45 phr, with 35 phr of carbon black and 10 phr ofsilica, guarantees an electrical resistivity of less than or equal to10⁶′Ω·cm, and a suitable thermal conductivity. In the example dealt withhere, the thermal conductivity of the tread wing is equal to 0.240W/m·K. The same elastomeric compound M3 is used for the two tread wingspositioned at the two ends of the tread, but the invention still remainsvalid if different materials are used. The required condition is to haveat least, at one of the two axial ends of the tread, an elastomericcompound with an electrical resistivity of less than or equal to10⁶′Ω·cm.

In a tire for a construction plant vehicle, the mass of the elastomericcompounds of the sidewalls is of the order of 15% of the total mass ofcompounds of the tire. The option selected by the inventors is to have alaminate of two sidewall layers to ensure both a low hysteresis and anelectrical conductivity of less than or equal to 10⁶′Ω·m. Combined withthe thickest and axially outer first sidewall layer is an elastomericcompound of low hysteresis with a viscous shear modulus of 0.125 MPa. Anelectrically-conductive elastomeric compound, with an electricalresistivity of the order of 10⁴⁴′Ω·cm, corresponds to the axially innersecond sidewall layer.

The results on tires were obtained by finite element calculations is inorder to determine the viscoelastic heat sources, the temperature andthe electrical resistivity.

Finite element calculations were carried out on the tires of theinvention and reference tires respectively. The results of calculations,for the reference tire, comprising a single sidewall layer (compoundM₂), and a tread (compound M₃) made of a single portion, are representedbelow in Table 3:

TABLE 3 Results Single sidewall layer Tread made of one portionElectrical 4.4 5.7 resistivity Log (Ω.cm) Viscoelastic 4520 5100 sources(W) Maximum 99.8 90 temperature° C.

The reference tire is electrically conductive with an average operatingtemperature of the order of 90.4° C.

For the tire of the invention, the results of the finite elementcalculations are summarized in Table 4:

TABLE 4 Axially outer Axially inner Central first sidewall second treadResults layer sidewall layer Tread wing portion Electrical 11.6 4.4 5.710.4 resistivity Log (Ω.cm) Viscoelastic 2080 580 696 3628 sources (W)Maximum 91.8 93.5 65.5 86.6 temperature° C.

The finite element calculations confirm the electrically insulatingnature of the axially outer first sidewall layer and of the centraltread portion. The tread wing in contact with the ground and the axiallyinner second sidewall layer are, on the other hand, electricallyconductive. The evaluation of the electric potential confirms theconduction pathway with levels of electrical resistivity ranging from10⁴′Ω·cm to 10⁶′Ω·m for the elastomeric compounds constituting thepathway for discharging the electrostatic charges.

For the tire of the invention, relative to the reference tire, theviscoelastic loss sources were halved in the sidewall of the tire, andin the tread the reduction is also significant.

As a consequence of the drop in the viscoelastic loss sources, thecalculation of the temperature field of the tire of the invention givesan average level of 92° C., which corresponds to a difference of 8%relative to the reference tire. This difference is sufficient for asignificant improvement in the endurance of the tire of the invention byprolonging its service life by around 30%.

The invention has been presented for a tire for a construction plantvehicle, but it can in fact be extrapolated to other types of tire.

1. A tire for heavy vehicle of construction plant type comprising: a tread comprising two axial end portions or tread wings axially separated by a central portion; two sidewalls connecting the tread wings to two beads, adapted to come into contact with a mounting rim by a bead layer comprised of electrically-conductive elastomeric compound; each said sidewall being axially on the outside of a carcass reinforcement comprising at least one carcass layer having metallic reinforcers that are coated in an electrically-conductive elastomeric coating compound; each said sidewall having a laminate comprising at least two sidewall layers that are at least partly axially superposed and having a total thickness E; the axially outermost first sidewall layer from said sidewall layers having a thickness E₁ and having a first elastomeric compound M₁; the first elastomeric compound M₁ having a viscous shear modulus G″₁ and a thermal conductivity λ₁; the axially innermost second sidewall layer from said sidewall layers having a thickness E₂ and having a second elastomeric compound M₂; the second elastomeric compound M₂ having a viscous shear modulus G″₂, a thermal conductivity λ₂ and an electrical resistivity ρ₂; each said tread wing having a third elastomeric compound M₃ having an elastic dynamic shear modulus G′₃ and an electrical resistivity ρ₃; wherein the thickness E₁ of the first sidewall layer is at least equal to 0.9 times the total thickness E of the laminate, wherein the thickness E₂ of the second sidewall layer is at least equal to the minimum value between 3 mm and 0.1 times the total thickness E of the laminate, wherein the first elastomeric compound M₁ of the first sidewall layer has a viscous shear modulus G″₁ at most equal to 0.165 MPa and a thermal conductivity λ₁ at least equal to 0.190 W/m·K, wherein the second elastomeric compound M₂ of the second sidewall layer has an electrical resistivity ρ₂ of less than or equal to 10⁶′Ω·m and a thermal conductivity λ₂ greater than that of the compound M₁ of the first sidewall layer, and wherein the electrical resistivities ρ₂ and ρ₃, respectively, of the second elastomeric compound M₂ of the second sidewall layer and of the third elastomeric compound M₃ of the tread wing are at most equal to 10⁶′Ω·m, so that the bead layer, the elastomeric coating compound of the carcass layer, the second sidewall layer and the tread wing constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground.
 2. The tire according to claim 1, the second sidewall layer being in contact via a radially outer top end with a said tread wing over a length L_(h), wherein the length L_(h) is at least equal to 10 mm.
 3. The tire according to claim 1, the second sidewall layer being in contact via a radially inner bottom end with the elastomeric coating compound of the carcass layer over a length L_(b), wherein the length L_(b) is at least equal to 10 mm.
 4. The tire according to claim 1, wherein the thermal conductivity λ₂ of the elastomeric compound of the second sidewall layer is greater than or equal to 0.240 W/m·K.
 5. The tire according to claim 1, wherein the third elastomeric compound M₃ of at least one said tread wing is an electrically-conductive rubber composition based at least on polyisoprene, on a crosslinking system, and on at least one reinforcing filler comprising carbon black, having a BET surface area at least equal to 110 m²/g, and a content at least equal to 30 phr and at most equal to 80 phr.
 6. The tire according to claim 1, wherein the second elastomeric compound M₂ of the axially inner second sidewall layer is an electrically-conductive rubber composition based at least on a mixture of polyisoprene and polybutadiene, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at least equal to 80 m²/g, and a content at least equal to 40 phr and at most equal to 60 phr.
 7. The tire according to claim 1, wherein the first elastomeric compound M₁ of the axially outer first sidewall layer has a rubber composition based on at least one blend of polyisoprene, natural rubber or synthetic polyisoprene, and polybutadiene, on a crosslinking system, and on a reinforcing filler, at an overall content at most equal to 45 phr, and comprising carbon black, at a content at most equal to 5 phr, and, predominantly, silica, at a content at least equal to 20 phr and at most equal to 40 phr.
 8. The tire according to claim 1, the central tread portion being formed by a fourth elastomeric compound M₄, wherein the fourth elastomeric compound M₄ of the central tread portion is a rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at most equal to 115 m²/g, and a content at most equal to 40 phr, and silica, at a content at most equal to 20 phr.
 9. The tire according to claim 1, the central tread portion being formed by a fourth elastomeric compound M₄, wherein the fourth elastomeric compound M₄ of the central tread portion is a rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler, at an overall content at most equal to 40 phr, and comprising carbon black, and silica.
 10. The tire according to claim 1, the central tread portion being formed by an elastomeric compound M₄, wherein the fourth elastomeric compound M₄ of the central tread portion is an electrically-conductive rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at least equal to 120 m²/g, and a content at least equal to 35 phr and at most equal to 80 phr, and silica, at a content at most equal to 20 phr. 